Surface plasmon resonance is an optical surface phenomenon that has been employed in sensors used in the fields of chemical, biochemical, biological or biomedical analysis. A surface plasmon is a surface charge density wave at the surface of a thin conducting film. A description of this phenomenon is found in the article by H. Raether in Physics Thin Films, 1977, 74 pp 237-244. This resonance can be observed when a polarized beam of monochromatic light is totally internally reflected from a dielectric interface having a thin metal film formed thereon. Usually the interface comprises a smooth surface of a transparent body such as glass. The light internally reflected by the interface has a minimum intensity at a particular angle referred to as a resonant angle in the literature. This angle is determined by the dielectric conditions adjacent the metal film and the properties of the film itself.
In prior sensors utilizing surface plasmon resonance, a thin metal film (SPR film) is usually applied to a flat surface of a glass prism. The resonance angle is determined by directing a polarized light beam through the prism onto the surface with the metal film thereon and measuring the intensity of the light reflected therefrom and through an external surface of the prism. Such arrangements, however, require a very high degree of precision in order to manufacture and align the separate optical parts so as to be able to produce accurate measurements. Additionally, such arrangements are not readily adapted to changing the prism to eliminate problems of contamination or to change the physical configuration to detect different materials.
The basis for the use of surface plasmon resonance for sensing is the fact that the oscillation of a surface-plasma of free electrons which exists at a conductor-dielectric boundary is affected by the refractive index of the material adjacent the conducting film surface on the side thereof opposite the prism. For a given wavelength of radiation, the resonance occurs when the angle of incidence of the polarized radiation has a particular value and this value is dependent on the refractive index of the material adjacent the film. As such, changes in the refractive index give rise to changes in the angle at which surface plasmon resonance occurs. When polarized light strikes the thin metal film at the "resonance angel", the intensity of the reflected light therefrom is minimized. Hence, by detecting the angle at which this minimum occurs, the refractive index of the material adjacent the film can be determined. The usefulness of this approach, however, has been limited due to system complexity related primarily to mechanical alignment issues associated with separate components.